The invention relates generally to metal oxide free fluoroelastomer compositions and to a fusing member having an outer layer of the fluoroelastomer for applying heat and pressure to fix toner particles to recording paper.
A fuser roll is a roller designed to apply direct heat and pressure to a toner image. The fluoroelastomer surface permits toner to be fixed to the recording paper without adhering to the roller surface and can withstand continuous exposure to high temperatures, silicone oils, toners, toner additives and paper product residue without unacceptable physical degradation.
In general, when forming images by xerographic processes, an image formed of a heat fusible toner powder is selectively disposed on a web-like surface of a recording medium, such as paper, by electrostatic forces. Toner powders are commonly formed of a mixture of thermoplastic and/or thermosetting resin carriers and additives such as amorphous carbon and magnetic particles. They are conventionally fixed to the recording paper by direct contact with a fuser roll which applies pressure and heat at temperatures between about 200.degree. to 400.degree. F.
The fusing process is conventionally accomplished by feeding a recording medium having the toner image thereon between the nip where two mated rollers meet. One or both of the rollers are heated, typically by an internal heat source within the roller, so that the surface temperature of the roller will be above the softening point of the resinous carrier of the toner.
The recording medium with the toner image thereon is fed between the two rollers which press towards each other to apply direct heat and pressure to the toner image. The amount of pressure and the length of time that the toner is heated determines the degree of fusing. The actual temperature range suitable for fixing toner images to recording paper is referred to as the "fusing window". The fusing window, TW can be defined by the formula: EQU TW=T.sub.OFF -T.sub.MIN
wherein T.sub.OFF is the Hot Offset temperature and T.sub.MIN is the minimum fusing temperature. Hot Offset temperature is the temperature at which the cohesive forces within the molten toner layer are less than the adhesive forces between the toner and roll surface so that toner adheres to the fuser roll. T.sub.MIN is the minimum temperature at which toner can be acceptably fixed to the recording paper. This temperature range is dependent on the raw materials, type of toner, release agents and the pressure applied by the roller. It is important that the toner is fixed without "offset" occurring, in order to produce copies of acceptable quality. For commercial applications, a fusing window of at least 30.degree. F. is acceptable for some applications. However, it is preferable to have as large a fusing window as possible. Thus, a 60.degree. F. fusing window is desirable and a 100.degree. F. fusing window is particularly ideal.
Conventional fusing systems have drawbacks. Softened toner generally has an affinity for the surface of the fuser roll it contacts. When toner adheres to the surface of a fuser roll, it can unintentionally be deposited on an unselected portion of the recording medium during the next rotation of the roll. This phenomenon is referred to as offset.
To prevent offset, a thin coating of a release agent such as polysiloxane fluid is commonly spread over the surface of the fuser roll which contacts the surface of the toner image. The polysiloxane fluid reduces the surface free energy of the roll surface and decreases the affinity of the toner for the roll. Surface tension values for several conventional fuser roll materials are set forth below in Table I.
TABLE I ______________________________________ Surface Tension of Fuser Roll Materials Fuser Roll Surface Surface Tension nMn ______________________________________ Polytetrafluoroethylene (PTFE) 18.0-18.5 Polyvinylidene fluoride (PVF.sub.2) 21-22 Polysiloxane Compounds 28-29 Polyfluorocarbon Elastomers 35-37 Polysiloxane Release Agents 19-21 ______________________________________
When compounding or formulating fluorocarbon elastomers, metal oxides are typically included to act as an acid accepter, cure activator, reinforcing filler and/or as an additive to promote improved chemical resistance. It is believed that every commercially available fuser roll having a fluoroelastomer surface contains metal oxide particles in at least the fluoroelastomer surface layer. U.S. Pat. Nos. 4,257,699, 4,264,181 and 4,272,179 describe fuser rolls in which additional metal oxide filler particles are added to the fluoroelastomer surface material to increase the metal oxide content of the surface of the fuser rolls beyond that needed to promote cure of the material. Fluoroelastomers described in U.S. Pat. No. 5,035,950 contain only so much metal oxide as is necessary to effect cure of the high fluorine content material.
These patents describe that use of a polymer release agent having mercapto functional groups applied to the surface of a fuser roll having metal oxide filler decreases problems associated with offset. When the metal-containing filler in the elastomer surface layer is present in sufficient amount, it interacts with the polymeric release agent to produce an active release film. This active release film prevents the thermoplastic resin toner from contacting the elastomeric material itself and accordingly, offset is avoided.
Although this active release construction has proven to be commercially acceptable, it nevertheless has drawbacks. Release agent fluids having mercapto functional groups are expensive. They can also present an unpleasant odor in the office environment and interfere with the ability to write or type on the copy sheet.
Accordingly, it is desirable to provide an improved fusing system which overcomes the shortcomings of the conventional fuser systems described above.